1. Field of the Invention
The present invention relates to a semiconductor energy detector using a charge coupled device (CCD), an active pixel sensor (APS), or the like.
2. Related Background Art
A charge coupled device (CCD) utilized as an image pickup device is a device for transferring a group of analog charges in one direction in synchronism with clock pulses, which can convert spatial information to time-series signals. However, if charges were transferred with the CCD being exposed to light, the transferred charges would mix with charges resulting from optical excitation in their respective elements, so as to cause so-called smear, thus degrading image signals. It is, therefore, common practice to use the device in time-sharing operation in division into charge storage periods for pickup of image (detection of energy ray image) and charge transfer periods for transfer of image. Practical imaging devices include, for example, frame transfer (FT) devices, full-frame transfer (FFT) devices, interline transfer (IT) devices, and so on. Among these, the FFT CCDs are mainly used for measurement. Since the FFT CCDs have no storage part and thus secure large photoreceptive part, they are high in efficiency of utilization of light and suitable for measurement of weak light.
Some semiconductor energy detectors such as the CCDs used for inspection of wafers or photomasks (reticles) or the like in the semiconductor fabrication field are required to have high sensitivity to ultraviolet light (for example, the g-line of high-pressure mercury lamp: wavelength 436 nm, the i-line of high-pressure mercury lamp: 365 nm, the XeCl excimer laser: 308 nm, the KrF excimer laser: 248 nm, the ArF excimer laser: 193 nm, etc.), because the inspection is carried out with a light source used for pattern printing exposure.
One of such image pickup devices is a back illuminated CCD (for example, Japanese Patent Application Laid-Open No. H06-29506). In a front illuminated CCD, transfer electrodes covering the photoreceptive part are, for example, electrodes of polycrystal silicon and image pickup is carried out using the front surface as a photoreceptive surface. However, since polycrystal silicon absorbs light of wavelengths not more than 400 nm, the sensitivity becomes lowered to the ultraviolet light. In contrast with it, the back illuminated CCD is the CCD whose substrate has, for example, the thickness as thin as about 10 to 30 xcexcm and which performs image pickup by injecting energy rays into the back surface. Therefore, it permits injection and reception of light without being affected by the transfer electrodes disposed on the front surface side, thus realizing the CCD with high sensitivity even to the short-wavelength light such as the ultraviolet light.
The back illuminated CCD as described above, however, has the problem that the mechanical strength thereof is low and it is difficult to handle because of the thinness of the membrane. In addition, while it is important to assure a substrate potential, i.e., keep the substrate resistance small in MOS devices including the CCDs, it is structurally difficult to realize the small substrate resistance in the back illuminated CCD having the large membrane.
Some back illuminated CCDs as described above are reinforced by a method of enhancing the strength by structure with a substrate frame, for example, as the one described in aforementioned Japanese Patent Application Laid-Open No. H06-29506. FIG. 12 is a bottom view from the back surface side of one example of such back illuminated CCDs and FIG. 13 a cross-sectional view along arrows VIxe2x80x94VI thereof. The cross-sectional view used in the description shows an end face thereof. In this example a membrane 2 is formed by thinning the substrate part corresponding to the photoreceptive region in the substrate 1 and a substrate frame 1a is formed by leaving the peripheral part in a frame shape without thinning the substrate. With increase in the area of the photoreceptive part, however, there will arise the problem that it becomes harder to assure sufficient mechanical strength of the membrane 2 by only this structure and distortion occurring in each part of the membrane 2 can cause an out-of-focus state (defocus).
Each pixel of the CCD is equivalent to a MOS-FET and creates or varies a potential well for transfer of charge of the CCD by accumulating or releasing charge of the substrate in response to a clock pulse applied to a gate. However, the substrate resistance is as low as about 0.7 xcexa9/xe2x96xa1 in the part of the substrate frame 1a where the thick substrate is left, whereas the substrate resistance is very high, about 100 to 1000 xcexa9/xe2x96xa1, in the membrane 2 because of its thinness. For that reason, the accumulation and release of substrate charge cannot be quickly done there. Particularly, though the substrate frame 1a also has the function of assisting the accumulation and release of substrate charge while enhancing the strength, the distance to the substrate frame la is long near the center of the membrane 2 corresponding to the photoreceptive part and thus the substrate resistance restrains the speed of charge transfer, so as to disable high-speed operation.
The present invention has been accomplished in view of the above problems and an object of the invention is to provide a semiconductor energy detector of the back illuminated type having sufficient substrate strength and small substrate resistance.
In order to accomplish this object, a semiconductor energy detector according to the present invention is provided as a semiconductor energy detector in which a charge readout area is formed in a front surface of a semiconductor substrate, wherein in a back surface of the semiconductor substrate there are a plurality of membranes formed by thinning the semiconductor substrate and arranged to detect an energy ray, a substrate frame formed in the peripheral part of the semiconductor substrate and embracing the plurality of membranes in an inside area thereof, and at lease one substrate beam formed in a border portion between the plurality of membranes, in the inside area of the substrate frame.
The detector may also be constructed so that the charge readout area is comprised of a charge coupled device.
When the back illuminated semiconductor energy detector provided with the membrane formed including the back-surface-side area corresponding to the photoreceptive area of the charge readout area such as the CCD formed on the front surface side of the semiconductor substrate and with the substrate frame as a left part of the substrate in the peripheral region is constructed in the structure in which the area of the membrane is divided into a plurality of regions and in which the substrate beam is formed by leaving all or part of the substrate in the border portion between those regions, it can be the semiconductor energy detector that has high sensitivity to the ultraviolet light and the like and that is capable of suppressing the defocus due to distortion occurring in each region of the membrane while assuring sufficient mechanical strength of the semiconductor substrate even in such cases that the area of the photoreceptive part is large.
When the substrate beam is constructed so as to adequately decrease the distance from each region of the membrane with high substrate resistance to the substrate frame or to the substrate beam with low substrate resistance, it becomes feasible to decrease the substrate resistance and implement the high-speed operation of charge transfer or the like.
The back illuminated semiconductor energy detector as described above can also be effectively applied to detection of energy rays such as an electron beam, X-rays, etc., in addition to the light including the ultraviolet light.
The detector may be one wherein the charge readout area is constructed to have a plurality of vertical channels dividing the horizontal direction and arranged with their longitudinal direction along the vertical direction and wherein the substrate beam is placed so as to pass a region on the back surface corresponding to at least two vertical channels.
Particularly, when the charge readout area is constructed to have a plurality of vertical channels, the detector can be constructed, for example, without any vertical channel whose entire area is an insensitive area, by placing the substrate beam so as to include the part corresponding to at least two vertical channels, i.e., by placing the substrate beam so as not to be parallel to the channel direction of the vertical channels.
Further, the detector may also be one wherein areas of regions where the plurality of membranes are provided, are equal to each other in regions on the back surface corresponding to the respective vertical channels.
As a method of imaging an object moving at a constant speed, for example, such as an article mounted on a belt conveyor, there is a TDI (Time Delay and Integration) driving method for accumulating charge while transferring the charge accumulated in the photoreceptive part, at a speed corresponding to the moving speed of the object. When imaging is conducted by applying this TDI driving method to the semiconductor energy detector in the above structure wherein the areas of the regions where the membranes are provided in the respective vertical channels, are equal, the detector can be constructed as one with uniform sensitivity for charge storage and detection corresponding to each position of the object throughout the entire image to be detected and picked up.
The above-stated TDI driving method is realized by structure further comprising a charge transfer control portion for controlling the charge readout area by the TDI driving method for carrying out detection without blurring of the energy ray image of the detected object while carrying out the charge transfer at the speed corresponding to the moving speed of the detected object, for example.